Yes Paul, whilst strips give an indication they are not accurate, the steps they go up in are just too broad to be useful. CYA stabiliser measurement is usually near hopeless and is best done with a turbidity type test.
The amount of water on strips varies according to the waters surface tension, how much you shake the strip or not and if the reagent water runs from one test blob to another.
I have found it ok in most cases to set the free chlorine level at 5% of the CYA level i.e. 50ppm CYA x 5% = free chlorine at 2.5ppm. It can go higher than this if required because some areas may need it and it can help when circulation is poor.
If you use the fine Desjoyaux filter bags you remove more dirt from the water and that means the chlorine has less to react with so it can last longer, similar to your fresh water fill. Sadly the standard Desy is more a strainer than a filter in the real sense.